DE 198 05 040 A1 describes a generic measuring device having a workpiece table for supporting a measurement object and a camera that is arranged above the workpiece table and that is displaceable vertically downwardly. Arranged below the workpiece table is a light source that illuminates the measurement object in what is known as transmitted light. The image recorded with the camera consequently shows the measurement object as a silhouette. However, more specific details of the measurement method are not described therein.
One example of a digital profile projector is described in United States Patent Application Publication No. 2010/0225666 A1. This profile projector generates a digital image of a measurement object, and a drawing of the measurement object is superimposed on the image display so as to be able to check whether the measurement object corresponds to the drawing within predetermined workpiece tolerances.
The imaging object of a real optical system and consequently also the camera of a real optical measuring device are known to always exhibit imaging aberrations that can be traced back in part to manufacturing tolerances and in part to trade-offs during the optical design and/or to underlying physical phenomena. The imaging aberrations represent the deviation of the real imaging optics from an ideal optical imaging that is possible only in theory. Typical imaging aberrations include spherical aberration, astigmatism, coma, image field curvature (focal plane deviation-FPD) and distortion. For increasing the measurement accuracy of an optical measuring device, the imaging aberrations in the image recorded by the imaging optics can be corrected by way of computation before, during or after the actual image evaluation, wherein correction values obtained in a previous calibration operation are used. U.S. Pat. No. 6,538,691 B1, for example, describes a computer-implemented correction of image distortions of a digital camera.
The known methods and measuring devices and the calibration methods used therein are not yet optimal, especially in cases where the imaging optics can be adjusted at different working distances from a workpiece table. The imaging aberrations are frequently minimized in optimum fashion only for a specific working distance.
Furthermore, the known methods and measuring devices generally proceed from a point image approximation in which for example the distortion is taken into account merely as a scale error in the imaging of a point, as a result of which higher-order imaging aberrations which affect the distortion are not taken into account. Furthermore, the known methods and measuring devices also do not take into account that even the image evaluation performed by software, for example in the form of edge evaluations, has an impact on the ascertained distortion of the measuring device.